Imaging systems are used in such fields as microelectronics, medicine, biology, genetic engineering, mapping and even astronomy. The imaging device can be a suitable type of microscope or, in the case of astronomy, a telescope. The demand for image accuracy is high and, therefore, the influence of noise in a signal derived by the imaging system from an imaged object must be minimized.
For reasons of convenience and efficiency, the invention will be described in the microelectronics environment, although another environment could also have been chosen. During the manufacture of very large scale integration (VLSI) semiconductor devices, measurements are made at several stages of the manufacturing process to determine whether particular features on the object are within specified design tolerances. If not, then suitable corrective action is taken quickly.
As is well known, such a manufacturing process produces a wafer which is divided into dies. Each die has a large number of electronic components. These components are defined by what can generally be termed “features” in the sense that a feature is detectable by a microscope as a foreground element distinguishable from a background, or vice versa, and having a dimension such as width. To measure that width the edges of the feature must be located accurately. “Edge” is a term used to signify detectable discontinuities in a signal obtained by imaging the feature (in any environment, not only microelectronics). The goal of edge detection is to accurately locate the transitions despite the influence of blurring and the presence of noise.
As technology has succeeded to increase the component density per die, the feature dimensions have shrunk to significantly below a micrometer. Consequently, the measurement equipment must measure submicrometer dimensions with lower allowable error tolerances.
Automated systems have been developed for making these measurements to replace manual systems in order to obtain higher process yields, to reduce exposure of the wafers to contamination and to provide a higher throughput. One example of an automated system is disclosed in U.S. Pat. No. 4,938,600. As shown in FIG. 1 which is taken from that patent, and explained in greater detail below, an image of a feature is recorded through a microscope and the recorded image is then processed electronically to obtain the required measurements. One such automated system is the Model IVS-120 metrology system manufactured by Schlumberger Verification Systems of Concord, Mass., a division of Schlumberger ATE Products. The major elements of the system, including a wafer handler, an optical system and a computer system, are mounted in a cabinet (not shown).
The wafer handler includes a cassette wafer holder 112 which contains wafers to be measured, a prealigner 114, a wafer transport pick mechanism (e.g., robotic transfer arms, not shown) for moving the wafers and a measurement stage 118 which holds the wafers during the actual measurement operation. During operation, the wafer transport pick mechanism removes a wafer 116 from cassette 112 and places it on prealigner 114. Prealigner 114 then rotates wafer 116 to a predetermined orientation by sensing a mark, a flat spot or notched edge on wafer 116, after which the wafer transport pick mechanism transfers wafer 116 from prealigner 114 to measurement stage 118 and positions wafer 116 in a horizontal orientation. Stage 118 is movable in three dimensions for precisely positioning wafer 116 relative to the optical system for performing the actual measurement.
The optical system includes microscope 120 and video camera 122 positioned above the measurement stage 118 and wafer 116. Microscope 120 typically has a turret carrying several objective lenses providing a desired range of magnification and is mounted so that microscope 120 and camera 122 have a vertical optical axis which is perpendicular to the wafer surface.
A feature to be measured on wafer 116 is located with microscope 120 in a well known manner by the movable measurement stage 118 until the feature is in the field of view of the objective lens. The optical system is focused, and a focused image of the feature is digitized and recorded by the camera 122. The image is then stored or “frozen”.
The system is controlled by a computer 130. Coupled to the computer 130 are a monitor 132 for display of the image recorded by the camera 122 and text, and a keyboard 136 (which constitute an input terminal for entering operator commands) and a disk drive 138 for storing system software and data.
Image processor 128 uses software algorithms to locate the edges of the selected feature and make a measurement. Computer 130 then displays the measurement data on screen, prints a hard copy or transfers the data directly to a host computer (not shown) for centralized data analysis. Once the process is complete, wafer 116 is returned to cassette 112 by the wafer handler.
One modification to the above system entails placement of the measurement stage in a vacuum inspection chamber which is maintained at vacuum pressure. Since the cassette 112 is usually in the ambient atmosphere, one or more chambers, often referred to as transfer chambers, are interposed between the ambient atmosphere and the inspection chamber for facilitating transfer of the wafers between the inspection chamber and the ambient atmosphere. The transfer chamber is alternatingly depressurized and repressurized. It is depressurized to the vacuum pressure in the inspection chamber, to enable transfer of an incoming wafer from the transfer chamber to the inspection chamber and transfer of an inspected, outgoing wafer from the inspection chamber to the transfer chamber. It is repressurized to atmospheric pressure to enable transfer of an inspected, outgoing wafer from the transfer chamber to the ambient atmosphere and transfer of an incoming wafer from the ambient atmosphere to the transfer chamber. To this end, gate valves are associated with each transfer chamber to isolate the vacuum environment from the ambient atmosphere during the transfer of the wafers between the inspection chamber and the ambient atmosphere. While in the transfer chamber, the wafers are usually placed on paddles or pedestals.
Typically, the inspection chamber is mounted on a vibration isolation platform which floats on an air cushion to thereby isolate the inspection chamber from environmental vibrations. The transfer chamber on the other hand is fixed to a stationary frame and since it must also be connected to the inspection chamber (to enable the transfer of wafers between the transfer chamber and the inspection chamber under vacuum pressure), vibrations of the transfer chamber are unavoidably transferred to the inspection chamber. Vibrations of the transfer chamber arise for example, during the depressurization and repressurization of the transfer chamber when pumps and valves associated with the transfer chamber are operated to provide for the desired gas flows. Such vibrations can interfere with the wafer inspection process and can cause movement of the wafers in the inspection chamber out of a position necessary for pick up by the robotic transfer arm for subsequent removal from the inspection chamber.
Thus, one problem arising from the placement of the measurement stage in a vacuum chamber mounted on an isolation platform and the coupling of the vacuum chamber to a fixedly mounted transfer chamber is the transfer of vibrations from the transfer chamber to the vacuum chamber.
One way to overcome this problem is to stop the inspection of wafers in the inspection chamber while the transfer chamber is operating, i.e., is being depressurized or repressurized. This, however, significantly reduces the throughput of the wafer inspection process since the transfer chamber and the inspection chamber are not operated simultaneously.
One proposed solution to avoid such a reduction in throughput is to make the transfer chamber relatively large to enable multiple wafers to be held therein. In this manner, the frequency of the depressurization and repressurization of the transfer chamber is reduced and the wafer inspection process could therefore be conducted for longer time periods. However, problems with this type of construction include the fact that the transfer chamber is quite large and occupies an excessive amount of precious space (space in a clean room in which the transfer chamber is situated is costly). Also, in view of the large volume of the transfer chamber, it takes longer to depressurize and repressurize. Thus, the problem of the reduction in throughput of the wafer inspection process is not entirely overcome.
Other constructions have, therefore, been sought to prevent the transfer of vibrations from the transfer chamber to the inspection chamber without reducing throughput of the wafer inspection process.
One solution, which not only prevents transfer of some vibrations from the transfer chamber to the inspection chamber but also seals the vacuum environment in the transfer chamber and inspection chamber from the ambient atmosphere, is to provide a coupling having a stiff, edge-welded metal bellows between the transfer chamber and the inspection chamber as shown in FIG. 1A. The bellows 140 surrounds a passage 142 between a transfer chamber 144 and an inspection chamber 146 through which wafers are passed. One edge of the bellows is coupled to the inspection chamber and the other edge is coupled to the transfer chamber. Since the metal bellows is oriented in an axial direction of the passage between the transfer chamber and inspection chamber, its axial ends are secured to the chambers and it is stiff when subjected to shear forces but yielding when subjected to axial compression. In other words, vibrations of the transfer chamber acting in an axial direction of the bellows are generally isolated by the bellows and not transferred through the bellows to the inspection chamber but vibrations of the transfer chamber acting in a transverse direction of the bellows are not adequately isolated in view of the particular construction of the bellows.
Moreover, the vibration isolation provided by the metal bellows is not entirely satisfactory because vibrations at frequencies of up to about 60-100 Hz can still be transferred from the transfer chamber through the bellows to the inspection chamber, and vibrations at these frequencies are not damped by the air cushion on which the inspection chamber floats.